1. Field of the Invention
The present invention relates to a tin-plated wire, particularly to a method for preventing the thermal oxidation yellowing thereof.
2. Prior Art
Conventional tin-plated wires have had a problem that they turn yellow due to oxide films produced by high temperature heating process to which they are subjected when they are being processed into wires for electronic components.
To eliminate the above-mentioned problems, the inventors of the present invention have studied the problem of such thermal oxidation yellowing of the tin-plating layer, through experiments by adding various kinds of metals to the tin-plating composition.
Therefore, it is an object of the invention to provide a method for preventing thermal oxidation yellowing of tin-plated wires.
It is another object of the invention to provide a tin-plated wire which is free from thermal oxidation yellowing caused by high temperature heating process.
In accordance with a first aspect of the invention, there is provided a method for preventing the thermal oxidation yellowing of a tin-plated wire, said wire comprising a core wire and a tin-plating layer formed on a surface of the core wire, wherein the tin-plating composition includes 30 to 500 ppm by weight of gallium, while the remainder thereof is tin or tin alloy.
In accordance with a second aspect of the invention, there is provided a method as set forth in the first aspect of the invention, wherein said tin alloy includes at least 0.5 to 5 percent by weight of copper, while the remainder thereof is tin.
In the case that the remainder is comprised of tin only, it has been found that no yellowing has occurred due to the oxide film produced by the heating of one hour at 210 degrees centigrade.
Further, in the case that the remainder is comprised of tin-copper alloy including 2.5 percent by weight of copper, in order to improve the properties of wires for electronic components and to lower a melting point thereof, it has also been found that no yellowing has occurred due to the oxide film produced by the heating of one hour at 210 degrees centigrade.
It should be noted that if the content of gallium is less than 30 wppm, then the effect for preventing the oxidation yellowing is diminished, while if it is more than 500 wppm, then the surface of the plating is roughened and thus its appearance is impaired. Accordingly, the desirable content of the gallium is in a range of from 30 wppm to 500 wppm.